Television camera tube



May 19, 1959 T. w. VAN lRLJssEL ETAL 2,887,595

f TELEVISION CAMERA TUBE Filed May 1o, 195e INVENTOR rus WILLEM VANmJssEL mrmus wlLLIsALous AuGusnNus soERs JOHANNES HENDRICUS JOSEPHUSMAARTENS JANLRIGTE BOERMAN AGENT TELEVISION CAMERA TUBE Tijs Willem vanRijssel, Martinus Willibaldus Augustinus Boers, Johannes HendricusJosephus Maartens, Jan Rigte Boerman, and Walter Heinrich Kuhl, all ofEindhoven, Netherlands, assignors, by mesne assignments, to NorthAmerican Philips Company, Inc., New York, N.Y., a corporation ofDelaware Application May 10, 1956, Serial No. 583,970 lClaims priority,application Netherlands May 14, 1955 9 Claims. (Cl. 31365) The inventionrelates to a television camera tube of the kind in which a chargepattern is produced by secondary emission. The tube comprises aphoto-electric cathode. Electrons set free from this cathode byprojecting a luminous image onto it are accelerated in the tube andfocused onto an image electrode, where they produce secondary emission,owing to which a charge pattern corresponding to the luminous image isproduced on the side of the image electrode facing the photoelectriccathode (image side) It is known that the image electrode of such acamera tube may be made of a substance which permits an electric chargeto 4leak from one side of the electrode to the other. The specicresistance of this substance should be within the range from 109 to 1012ohm-cm. To this end so-called conductive glass is particularly suitable,an example being the glass known in the United States of America as G-Bglass, exhibiting a specific resistance of about 5 X 101 ohm-cm.

It is advantageous to have a high secondary-emission coefficient on thesurface of the image electrode on the image side. It has therefore beensuggested to provide the image electrode on the image side with a thinlayer of an alkali oxide or alkaline earth oxide to increase thiscoefficient. However, it has been found that the improvement obtained bysuch an oxide layer on conductive glass is not always persistent andthat in some of the known tubes the increase in efficiency obtained bysecondary emission gradually falls olf.

This phenomenon becomes manifest with an increase in the collectoraccelerating voltage, i.e. the voltage at which the secondary electronsare conducted away as completely as possible to the collector. Theinvention provides means to obviate this disadvantage and to obtain ahigher efficiency and longer lifetime of the tube.

In a television camera tube according to the invention, which tubecomprises cesium and is provided with an image electrode of conductiveglass, on which secondary emission produced by electrons emanating froma photo-electric cathode produces a charge pattern corresponding to aluminous image projected onto the photoelectric cathode and which isprovided on the image side with a thin secondary-emissive layer(referred to hereinafter as the coating), this layer consists mainly ofan inorganic sodium or lithium-halogen compound.

It is known that layers of an oxide or a halogenide of an alkali, analkaline-earthor an earth metal on a metal electrode exhibit acomparatively strong emission.

It is furthermore known that the insulating image electrode of a cameratube operating with an electron beam focused on this electrode, where itsets free secondary electrons, may be constituted by an insulating layerof chloride or bromide of one or more alkali metals, for example KCl orNaCl, which is applied to a metal electrode. If necessary, an insulatinglayer for example of mica may be sandwiched between the said layer andthe metal electrode.

Patented May l19, 1959 ice t A camera tube has furthermore beendescribed, of which the image electrode is constituted by a thininsulating or semi-conductive layer, which may be made of an alkalihalogenide and which is metal-coated on the rear side. This imageelectrode is scanned on the metalcoated side by rapid electrons whichmust render the image electrode conductive.

In none of these known cases, in w'hich reference is made to halogenidecompounds, is there concern with the transport of electric chargethrough conductive glass, which is coated with 4a secondary-emissivelayer, this being in contradistnctionto the tube according to theinvention in which the said layer contains sodiumand/or lithium atoms orboth. The fact that with this tube suchfsurprising results are obtainedcould be accounted for by the following theory, whichis not, however, tobe considered as limiting.

It is presumably ions, emanating from the coating, that transport thecharge through the glass. It might be that ions of the monovalentelements sodium and lithium are particularly suitable thereto, since,owing to their small size, they are suliiciently movable in the glass.

The invention may be carried into effect by using as substance for thecoating a chlorine compound, such as sodium chloride.

Further development of the invention has showny that particularly uorinemay be selected as the halogen and that cryolite (sodium-aluminumuoride) is to be considered as an extremely favourable substance for thecoating.

In accordance with a further aspect of the invention, atoms of atrivalent, ilumine-binding metal are applied to the coating containing afluoride compound, the quantity of this metal being such that thesurface conductivity and the secondary-emission coeicient of the coatingare substantially not affected and that the majority of the fluorineatoms added initially to the sodium or lithium atoms in the coating arebound. It is thus ensured that during the operation of the tube freeiiuorine atoms produced by secondary emission of the coating are boundwith certainty in the coating, so that they do not emerge from thecoating and Wander through the tube. Trivalent metals suitable to thisend are, for example, antimony, bismuth, indium and arsenic. Use ispreferably made of antimony. The atoms of this metal form, on thecoating, a compound with the available cesium atoms, so that a compoundhaving a comparatively low electrical conductivity is obtained. Even inthe case of a comparatively large quantity of antimony on the coatingthe surface resistance thereof is little affected. A further advantageof antimony is that it hinders little the movement of the primary andsecondary electrons striking the coating.

The invention will be described more fully with refer- 'ence to thedrawing, which shows diagrammatically one for example, a cryolitecoating 15 to enhance the secondary emission of the image electrode.

The part having the smaller diameter is closed by a at ,glass bottom 6,to the inner side of which is applied thephotorelectric cathode 7. Thecathode may, as usual, be made of antimony activated by cesium.Reference numeral 8 designates an optical system, by means of which aluminous image can be projected onto the photoelectric cathode. Y

To the coating 1S is applied by vaporisation a small quantity ofantimony. The antimony may be applied directly after or, if desired,partly simultaneously with the Vapplication 'by vaporisaton of thecryolite of the coating 15. Tlhe quantity of antimony applied is about1% by weight of the applied quantity of cryolite in the coating 15. Thisquantity of antimony does not introduce an kappreciable change of theelectrical surface re- `sistance of the coating 15, so that the imagesharpness is not affected. The secondary-emission coeticient ofthacoating appears to be `slightly higher owingto the antimony; this isbelieved to be due to a reduction in emission potential, owing totheformation of an antimonycesium compound on the surface of the coating1S, the cesium, of course, originating from the photo-cathode 7.

In the side tube 9 is arranged an electrode system 10, which serves toproduce an electron beam of definite intensity. The axis of this systemis directed to the centre of the image electrode 5. By means of theconventional deflection coils 11 and 12 the path of the electron beamcan be controlled and by suitable energization of the coils 11 and 12the complete image surface of the image electrode is periodicallyscanned bythe electron beam.

yThe wall of the tube has a conductive coating 13, which is electricallyconnected to an annular collector 14, ar-

ranged in the proximity of the image electrode.

`The image electrode is formed by a conductive glass layer of about 60p.applied to the metal supporting plate 4. The` specic resistance of thisglass is about 1011 `ohm-cm. under the conditions of operation of thetube.

On the image side the conductive glass is coated with a layer ofcryolite 15 of about 500 A. in thickness, precipitated from the Vapourphase.

V- On the wall inside the side tube 9 provision is made of an annulargold layer 16. This layer serves as a store for, or supply of, cesium,but provides at the same time that no excess of cesium vapour iscontained in the tube, which wouldV be harmful to the sensitivity of thephoto-electric cathode. The cesium is adsorbed by Vthe gold layer whencesium vapour is introduced into the tube during manufacture. The goldlayer keeps the cesium away from the electrode system 10.

H The current conductor 3 is connected through a load resistor 17 to thenegative terminal of a direct-current source 18, the positive terminalof which is connected `to the coating 13 and hence also to the collector14, so

tial at the photo-electric cathode 7 with respect to the .coating 13 andthe collector 14. The potential difference between the collector and theelectrode system `with the photo-electric cathode is high with respectto the diierence between the collector andthe supporting plate 4, sothat both the electrons of the scanning beam pand, the electronsemanating from the photo-electric cathode strike the cryolite layer witha suicient velocity to set freeA secondary electrons from this layer.lWhenstruclcby lthe scanning beam, each point of the surface of theimage electrode is brought to a potential Vwhich is'approximat'ely equalto that of the collector.

During the time interval between two successive scanjnings (frameperiod) the potential of the point is reduced .since 'the imageelectrode is traversed by electric charge.

Owing to this potential drop secondary electrons set free byjthephoto-electrons during the frame period can be attracted and collectedby the collector 14. Consehas a certain amount of conductivity.

The secondary-emission coeflicient of the cryolite layer 15 isparticularly high.` Values between 9 and 11 are found. These high valiesare also ascribed to the presence of cesium in the tube. Cesium `adheresreadily to antimony and `halogenide ions, particularly to those offluorine, which substance is contained in the cryolite. For a large partthe favourable properties of the tube according to the invention may bedue to these conditions.

It is assumed that `cesium atoms adsorbed at the surface of the coatingcould then bind halogenide atoms, while forming ions, these halogenideatoms being produced wherrhalogenide ions give off electrons bysecondary emission.` This bond could prevent the halogenide atoms fromleaving the coating and from spreading in the tube, and from alectingthe photo-electric cathode.

Consequently, the `tube should contain cesium. In the Vembodimentdescribed this substance is provided in the photo-electric cathode. Ifthis is not the case, a sepa- `rate supply ofcesium` can be provided.This provision may, otherwise, also be made when the photo-electriccathode already contains cesium, since experiments have shown that anexcessively small supply of cesium in the tube may imply that after sometime such a quantity of cesium may be withdrawn from the photo-cathodethat' the effect of this cathode falls ctt. In the embodiment describeda cesium supply is formed by the gold layer 16 inthe side tube `Whenthecoating of the tube according to the invention contains fluoine, asis the case with the example described, the spreading of fluorine atomsaffecting the photo-electric cathode in the tube `is avoided withcertainty by applying to the coating by vaporisation a small quantity ofatoms of a trivalent, ilumine-binding metal. Such a metal may, forexample, be antimony, bismuth, indium or arsenic: use is preferably madeof antimony, as is indicated above in the description of the example.

Camera tubes for television exhibit rather often a troublesomephenomenon, referred to as inscription This phenomenon consists inthatduring the use of the tube the coeflicient of the secondary emission ischanged at those areas ofthe image electrode which have been loadedmost. This drawback may sometimes be obviated or at least reduced bytaking the tube out of operation for some time.

Cryolite has the favourable property that it does not or substantiallynot exhibit the aforesaid phenomenon of inscription Since it is,moreover, readily precipitated from the vapour phase in the tube itselfon the con ductive glass, it is an excellent material for themanufacture of the secondary-emissive coating.

What is claimed is:

l. A television camera tube comprising an envelope and inside saidenvelope the combination comprising a photo-electric cathode and imageelectrode cooperating to establish a charge pattern on said imageelectrode corresponding to a televised object, said image electrodeincluding a conductive glass member and a thin,secondary-electron-emissive coating thereon comprising essentially aninorganic halogen compound containing an alkali metal selected from thegroup consisting of sodium and lithium, Said tube also containingcesium.

2. A` television camera tube as set forth in claim l wherein the halogencompound is a uorine compound.

3.V A television camera tube as set forth in claim 2 wherein theemissive coating consists essentially of sodium-alu`rninum fluoride.

4. A television camera tube comprising an envelope and inside saidenvelope the combination comprising a photo-electric cathode containingcesium and an image electrode cooperating to establish a charge patternon -saidirnage electrodecorresponding to a televised object; `said imageelectrodeincluding a glass member having a resistivity within the rangefrom 109 to 1012 ohm-cm..

an electrode on one side of said glass member, and a thin,secondary-electron-emissive coating on the other side of said glassmember and constituted of an inorganic halogen compound containing analkali metal selected from the Igroup consisting of sodium and lithium;a supply of cesium within said envelope and spaced from saidphoto-electric cathode; and means for scanning said other side of theimage electrode with an electron beam.

5. A camera tube as claimed in claim 2 wherein the emissive coatingcontains on a surface atoms of a trivalent, uorine-binding metal in anamount such that the surface conductivity and the secondary-emissioncoeficient of the coating are substantially not aiected.

6. A camera tube as claimed in claim 5 in which the trivalent metal isantimony.

7. A television camera tube comprising a photo-electric cathodecontaining cesium and an image electrode, said image electrode beingconstituted of a glass member having a resistivity in the range of 109to 1012 ohm-cm., and a thin cryolite coating on said glass member.

8. A tube as set forth in claim 7 wherein antimony is present onthecryolite coating.

9. A tube as set forth in claim 8 wherein the antimony constitutes about1% by weight of the cryolite, and the cryolite coating has a thicknessof about 500 A.

References Cited in the tile of this patent UNITED STATES PATENTS2,535,817 Skellett Dec. 26, 1950 2,547,638 Gardner Apr. 3, 1951 FOREIGNPATENTS 510,784 Canada Mar. 8, 1955

